In typical modern wireless transceivers the communication frequency is derived from a reference oscillator. For example, in a wireless hand-set a frequency based on the reference oscillator is used to initiate the communication with a base station. After the connection has been established, the frequency accuracy may further be improved by various synchronization methods, but the initial communication frequency must be accurate enough to enable the establishment of the initial communication.
In a typical transceiver architecture, the output of a reference oscillator is phase-locked to the output of a VCO and the output of the VCO provides the desired local oscillator (LO) frequency.
An example of such PLL based frequency generation is depicted in FIG. 1a. The output 13 of the phase comparator 12 is used to stabilize the operation of the VCO 14; the output frequency 15 of the VCO 14 will be the reference oscillator frequency 11 multiplied with the dividing factor N of the divider chain 16.
The basic PLL based frequency generation utilizes an integer counter chain as frequency divider, but the fractal version uses a fractal divider which alternates between two different dividing modulo, typically under control of a sigma-delta converter, to approximate a continuous output frequency range.
The PLL based frequency generation depicted in FIG. 1a can be developed into the frequency synthesizer depicted in FIG. 1b if the dividing factor N is arranged to be dynamically changeable.
In addition to the requirement to provide long-term frequency accuracy for the VCO, a low reference oscillator phase noise is needed because the less phase noise a LO generated carrier contains, the better the communication channel will be suitable to provide error free information transfer.
Reference oscillators are used as precise and stable frequency references in frequency synthesizers and to provide a precise and stable reference frequency signal to enable the generating of a variable but stable frequency that can be used as a tunable LO-frequency.
The critical requirements for the output frequency signal of a frequency synthesizer is stability, low phase noise and a high thermal stability, e.g. having low thermal coefficient, and additionally the requirement that a precise value for the output frequency can be selected rapidly.
In wireless communication devices the reference oscillator has conventionally been based on quartz crystals. The stable and precise mechanical vibration of quartz resonators suits well for creation of an oscillator that has excellent long term (drift and aging) and short-term (phase noise) stability. Furthermore, by proper quartz preparation methods (e.g. AT-cut) the temperature dependence of the resonance frequency can be reduced to a low value (less than a few ppm for the typical operation temperature range). The central disadvantages of quartz crystal reference oscillator modules are their rather bulky size and difficulty for monolithic integration with the transceiver module that is otherwise typically based on highly integrated solutions. Modern micromachining makes it possible to fabricate minituarized mechanical resonators (MicroElectroMechanical Systems=MEMS) with resonance frequencies ranging from several kHz up to the GHz range. Examples of such microresonators based on surface or bulk micromachining of silicon are presented in H. J. De Los Santos, “RF MEMS Circuit Design for Wireless Communications”, Artech House, Boston/London, 2002. The advantages of microresonators include small size, low power consumption, and possibility for increased integration level between the resonator, the oscillator electronics, and the device package. Both monolithic integration and the system-on-chip approach are viable solutions for increasing the integration level of a reference oscillator. Monolithic integration of micromachined resonators and integrated circuits facilitates more complicated microelectro-mechanical circuits, and can provide complete on-chip frequency synthesizers.
However, a fundamental complication in using silicon-based microresonators in frequency synthesizers arises from their large temperature coefficient, typically the df/dT from −10 to −30 ppm/K. Such a temperature dependence is far too large for a reference application if left unaccounted for. Compensation of this temperature dependency is therefore required to make microresonators suitable as frequency references for frequency synthesizers.